Rotary lithographic presses are well known in the art, as is the requirement to dampen a printing plate of the press in order to prevent or retard contamination of the resist areas of the printing plate with oil based printing inks used in printing from the plates.
Such plates include a dot matrix of up to 100%, which has been rendered oleophilic and hydrophobic, the remaining areas of the plates being hydrophilic and oleophobic.
Numerous devices have been proposed for effecting the dampening of the hydrophilic areas of the plates in order to resist contamination of those areas by the printing ink employed in the printing process. However, none of these devices has been entirely successful in eliminating "scuffing" of the printing plate, and in the elimination of "gear markings" in the finished print.
The undesired effect of "scuffing" arises when the resiliently compressible form roll is distorted out of round by a compressive force exerted thereon by the relatively incompressible printing cylinder. As the printing cylinder itself is cylindrical, such pressure results in an axial indentation in the form roll, the form roll being formed from a resiliently compressible rubber-like material. This manifests itself as an increase in the peripheral speed of the form roll as related to the peripheral speed of the printing cylinder, and in turn results in scuffing of the printing plate, i.e., a smearing of the ink dots forming the dot matrix in the direction of rotation of the printing cylinder.
The term "gear markings" is one commonly used in the trade, and relates to a smearing of the printing ink applied to the printing plate arising from differences in the peripheral speeds of the printing cylinder and, that of a form roll, and which manifests itself as transverse lines of smearing in the finished print.
While scuffing can be reduced to acceptable limits by backing-off the form roll in order to decrease the pressure imposed thereon and reduce the distortion of the form roll to an out-of-round condition, this, in turn, results directly in play or chatter in the gearing employed to drive the form roll and the pick-up roller from the printing cylinder. Chatter or play in the gearing results in oscillating variations in the peripheral speed of the form roll, and intermittent and cyclical over-speeding of the form roll, or, under-speeding thereof. This is seen by the printing cylinder and its printing plate as a variable scuffing of the printing plate in forward and reverse directions, and, manifests itself as lines of transverse smearing in the printed image.
Loudon U.S. Pat. No. 4,455,938 issued June 26, 1984 is typical of such a construction in its use of a direct gear drive from the printing cylinder to the form roll, and a direct gear drive from the form roll to a metering roller. In order to adjust the pressure at the nip between the form roll and the printing cylinder, the form roll must be adjusted or "backed off" radially of the printing cylinder. Similarly, to adjust the pressure at the nip between the metering roller and the form roll, the metering roller must be adjusted or "backed off" radially of the form roll. Such adjustments either can result in the bottoming of the teeth of the associated drive gears and the feeding of noise and vibrations to the form roll, or, in the alternative, can result in free play and chattering in the gear teeth resulting in overspeeding and underspeeding of the form roll. Both of these conditions can result in "scuffing" and "gear marking" in the finished print.